Each supported platform has a kernel Performance Metrics Domain Agent (PMDA) which extracts performance metrics from the kernel of that
platfrom. A variety of platform-specific metrics are available, with an equally varied set of access mechanisms - typically this involves
special system calls, or reading from files in kernel virtual filesystems such as the Linux sysfs and procfs filesystems.
The platform kernel PMDA is one of the most critical components of the PCP installation, and must be as efficient and reliable as possible.
In all installations the default kernel PMDA will be installed as a shared library and thus executes directly within the pmcd(1) process.
This slightly reduces overheads associated with querying the metadata and values associated with these metrics (no message passing is
required).
Unlike many other PMDAs, the kernel PMDA exports a number of metric namespace subtrees, such as kernel, network, swap, mem, ipc, filesys,
nfs, disk and hinv (hardware inventory).
Despite usually running as shared libraries, most installations also include a stand-alone executable for the kernel PMDA. This is to aid
profiling and debugging activities, with dbpmda(1) for example. In this case (but not for shared libraries), the following command line
options are available:
-d It is absolutely crucial that the performance metrics domain number specified here is unique and consistent. That is, domain should
be different for every PMDA on the one host, and the same domain number should be used for the same PMDA on all hosts.
-l Location of the log file. By default, a log file named [platform].log is written in the current directory of pmcd(1) when pmda[plat-
form] is started, i.e. $PCP_LOG_DIR/pmcd. If the log file cannot be created or is not writable, output is written to the standard
error instead.
-U User account under which to run the agent. The default is the unprivileged "pcp" account in current versions of PCP, but in older
versions the superuser account ("root") was used by default.

INSTALLATION

Access to the names, help text and values for the kernel performance metrics is available by default - unlike most other agents, no action
is required to enable them and they should not be removed.

FILES

$PCP_PMDAS_DIR/[platform]/help
default help text file for the the kernel metrics
$PCP_LOG_DIR/pmcd/pmcd.log
default log file for error messages and other information from the kernel PMDA.

PCP ENVIRONMENT

Environment variables with the prefix PCP_ are used to parameterize the file and directory names used by PCP. On each installation, the
file /etc/pcp.conf contains the local values for these variables. The $PCP_CONF variable may be used to specify an alternative configura-
tion file, as described in pcp.conf(5).